In general, a gasket is used to seal a fluid such as water, oil, vapor and gas in equipment, joints or the like of various pipes in a high-temperature and high-pressure state in petroleum refining, petrochemical plants, LNG plants, power plants, steel works, etc. As mentioned above, a gasket is required to exhibit excellent sealing property in a high-temperature and high-pressure state or under severe heat cycling and temperature cycling conditions.
As a gasket, a spiral-wound gasket, a serrated gasket or the like are known.
A spiral-wound gasket is obtained by winding a hoop and a filler in a stacked state. As such spiral-wound gasket, one having a basic configuration provided with a gasket main body in which the both sides thereof serve as a pressure-receiving surface that contacts the sealing surface; or an inner-ring attached, an outer-ring attached, or an inner-and-outer-ring attached one in which a metal reinforcing ring is provided on the inside, the outside or both sides of a gasket main body in order to prevent deformation of a gasket main body inwardly or outwardly by a clamping or to center the gasket main body are known.
In a serrated gasket, generally, a number of concentric circular grooves having different diameters are drilled on both surfaces of a metal main body at almost equal pitches in the radial direction, whereby the cross section has a sawtooth shape. Due to such a shape, a high surface pressure is generated on the front end portion of a sawtooth even the clamping surface pressure is low. Further, since it becomes possible to set the width (length in the radial direction) of a gasket to be narrow, a serrated gasket is suited as a gasket for equipment such as a heat exchanger in which the width of a gasket seat in a flange is narrow.
Patent Document 1 discloses a spiral-wound gasket in which expanded graphite is used as a filler. A sealing material formed of expanded graphite has sufficient elasticity and is improved in heat resistance. However, as for expanded graphite, in a temperature range exceeding 500° C. in the presence of oxygen, disappearance by oxidation of expanded graphite is promoted. Therefore, it was difficult to maintain stable sealing property for a long period of time. Patent Document 2 discloses a spiral-wound gasket in which unexfoliated mica and expanded graphite are used as a filler. However, in this spiral-wound gasket, expanded graphite disappears when used at high temperatures, and hence, sealing property cannot be maintained. Patent Document 3 discloses a spiral-wound gasket in which unexfoliated mica is used as a filler. This gasket was poor in sealing property and only a sheet having a high density could be obtained.